1. Field of the Invention
The invention generally relates to backlights for liquid crystal displays, and more particularly, to a backlight system providing a constant brightness.
2. Description of the Related Art
Liquid crystal displays (LCD) are commonly used in portable computer systems, televisions and other electronic devices. An LCD requires a source of light for operation because the LCD is effectively a light valve, allowing transmission of light in one state and blocking transmission of light in a second state. Backlighting the LCD has become the most popular source of light in personal computer systems because of the improved contrast ratio and brightness. LCDs have become especially popular in portable computer applications because they are sufficiently rugged and require little space to operate.
Backlighting is generally provided to LCDs using a fluorescent lamp and some means for diffusing the light generated by the lamp to create a uniform pattern of light behind the LCD. A preferred diffusion technique is shown in U.S. Pat. No. 5,050,946 entitled "Faceted Light Pipe." The intensity of the light generated by a fluorescent lamp generally depends upon the current through the lamp and the lamp's temperature. Constant current or input voltage feed forward supplies have conventionally been used to ensure that the backlight current remains steady, so that the brightness remains relatively steady.
When a fluorescent lamp first receives power, however, it is generally cold. Cold fluorescent lamps generally provide relatively little light, and generate increasing light as the temperature increases. Consequently, when the computer system is first turned on, the display often appears unusually dim. To improve the display's readability, the user frequently adjusts the brightness control. As the fluorescent lamp warms up, the intensity of the light generated by the lamp increases. This increase is so gradual, however, that the user's eyes often adjust and the user is unlikely to notice the increased brightness.
If the user happens to notice the increased brightness, he is likely to adjust the contrast instead of the brightness to improve the display's readability. Although adjusting the contrast changes the apparent brightness of the display, the actual brightness of the lamp is not affected. Instead, the ratio of the luminance values for the foreground and background on the display is changed. Consequently, adjusting the contrast on an LCD does not affect the current through the lamp, so the current drain on the battery in the computer system is higher than if the brightness had been adjusted. As a result, the unnecessary brightness of the lamp reduces the battery life for the entire system.